Recently, SiC as a material for a power device, which provides high electric field breakdown strength, is brought to attention. Since a SiC semiconductor device has high electric field breakdown strength, the device can control large current. Thus, the device is expected to apply a control of a motor for a hybrid vehicle.
In the SiC semiconductor device, it is effective to increase a channel density for flowing large current. Accordingly, in a silicon transistor, a trench gate structure MOSFET is utilized and put to practical use. The trench gate structure is also applied to the SiC semiconductor device. However, when the structure is applied to the SiC, it has a large difficulty. Specifically, the SiC has the breakdown electric field strength higher ten times than silicon. Thus, the SiC semiconductor device is utilized under a condition that a voltage higher ten times than a silicon device is applied to the SiC semiconductor device. Thus, the electric field having strength higher ten times than the silicon device is applied to a gate insulation film, which is formed in a trench formed in the SiC. Thus, it has a difficulty such that the gate insulation film at a corner of the trench is easily damaged. When calculating with simulation under a condition that 1200 volts is applied to the drain, the electric field of 10 MV/cm is concentrated at the trench gate. It is necessary to reduce the electric field to be equal to or lower than 5 MV/cm, which is a half of 10 MV/cm in order to utilize actually.
To solve the above difficulty, Patent Literature No. 1 proposes a SiC semiconductor device, in which a P type layer is formed on a bottom of a trench, which provides the trench gate structure. Thus, when the P type layer is formed on the bottom of the trench, which provides the trench gate structure, the electric field concentration in the gate insulation film on the bottom of the trench is reduced, so that the gate insulation film is prevented from being damaged. Further, if the P type layer is formed on only the bottom of the trench, a surge may penetrate into the bottom of the trench gate structure and damage the gate insulation film when the surge occurs. Accordingly, the P type layer is formed also between adjacent trenches, so that an equipotential line is prevented from being penetrated between the trenches when applying a reverse bias, and the gate insulation film is prevented from being damaged.
However, since the SiC is a material having a wide gap, an inner electric potential of the SiC is equal to or larger than 3 volts, which is large. Accordingly, even when a source and/or a drain is connected to a zero volt ground, a depletion layer, which is prepared by applying minus three volts to the P type layer, expands naturally. Accordingly, the depletion layer expanding from the P type layer narrows a current passage between P type layers, and therefore, a difficulty arises that the on-state resistance increases.